Lowtech Graph lists low-tech options for every need of a home. Here you can compare them, find tutorials to build them, and link them together into a circular system. Why low-tech?
Heat that stays in the house in winter instead of leaking out, thanks to insulation, airtightness, better glazing and treated thermal bridges. It is the cheapest heat there is; how much a measure saves depends on the house, so it is not quantified.